The present invention relates to the field of electronic motor control and, more specifically, to a method and apparatus for using back EMF to power electrical circuits and retract a servo arm.
Magentic disk devices, such as hard disk drives supporting random access, utilize a spindle that includes a collection of platters. These platters are covered with a magnetic material for recording information. Each platter contains a series of circular recording tracks containing sectors of information that can be read or written to by electromagnetic heads utilizing switchable magnetic fields. The platters of a spindle generally rotate at a constant angular speed when memory sectors are being read or written.
The rotation of a spindle of platters in a hard disk drive is effected by a spindle motor. The motor includes a magnetic rotor rotating in response to an electrical field created by three sets of electric coils.
A disk drive can be spinning with the read/write head out over the data area when there is a power fault. The fault can be as severe as a complete loss of power to the drive. In such a case the servo arm must be retracted to the landing zone before the spindle loses a significant amount of speed. If the arm is not retracted, it can contact the data area and damage either the surface of the disk, the sensor at the end of the arm, or the arm itself.
Some disk drives are manufactured with a latch that locks the head in the retract position once the disk speed drops below a specified minimum level. In some drives a magnet holds the arm in the landing zone. There are other devices that are used to perform this same function. It is desirable to quickly reduce the spindle speed once the arm is in the landing zone regardless of the type of mechanism used to keep the arm there.
Electrical energy is used to start up and maintain the rotation of the spindle in a disk drive. An electrical motor provides the required torque. The momentum that exists can be converted back to electrical energy if the power supply is not available. In order to take advantage of this source of energy, the motor needs to be isolated from the power supply. If it were not isolated, loads connected to that node could quickly drain the motor. When the supply is providing current for the motor in normal operation, the isolation device needs to have low on resistance to minimize power dissipation.
The present invention reduces the inrush current to the supply capacitor because that capacitor can be relatively small and thus prevents damage to the forward biased diode associated with an isolation device. Additionally, partial braking of the spindle occurs during the retract period. The retract operation of the present invention can occur while back EMF is at a low voltage when the spindle has slowed to a small speed. The present invention used a pulse signal generated by a flyback voltage to achieve a higher voltage to be used in conjunction with the small capacitor.